Monochromators generally comprise an entrance slit for receiving a source of light to be analyzed or filtered, means for separating the light into its individual component wavelengths and an exit slit for selecting out a desired component. More particularly, monochromators of one class comprise an entrance slit, a mirror for receiving the light output by the entrance slit and collimating it, a grating for dispersing the light into its individual components, a focusing mirror for receiving those components and refocusing them into the image of the original slit for presentation at an exit slit.
Because of the nature of the optical elements involved, it is necessary that the same be arranged in an off-axis configuration. By this, it is meant that it is not possible to position the elements in a way in which the normals to all the optical elements lie along a single straight line. While, in principle, an on-axis system can be designed using lenses and a prism instead of mirrors and a diffraction grating, transmissive systems do not provide acceptable performance levels.
A necessary defect of such off-axis arrangements is the introduction of relatively severe aberrations into the system, which aberrations consist mostly of coma and astigmatism. As can be understood from the above, the magnitude of the coma bears a direct relationship to the magnitude of the off-axis angle. More importantly, the coma component also has a sign which is also a function of the off-axis angle. As is well known in the prior art, it is possible in the case of an instrument having two offaxis mirrors to adjust the off-axis angles to have opposite orientations and magnitudes which result in one coma being subtracted from the other.
One such monochromator is the so-called Czerny-Turner type monochromator. Generally, a Czerny-Turner monochromator comprises an entrance slit (serving as a source) which illuminates a first concave collimating mirror which in turn reflects the light onto grating. The grating disperses the light into its individual components and reflects those components to a focusing mirror. This mirror focuses an image of the entrance slit onto an exit slit for that component of the input light which has a wavelength which corresponds to the angular orientation of the grating.
While this does solve a portion of the problem, the absolute cancellation of coma is only possible in the zero order which, of course, is of little interest in a monochromator. It is noted that, within the zero order, the reflected light leaves in the same direction as the incident light and thus has the same width. On the other hand, if the reflected light leaves at a different angle as it does in the case of the first and higher orders, it has a width which is a function of that angular difference. This results in a degradation of the performance of the monochromator. This general problem is referred to as the anamorphic effect. It is also noted that the anamorphic effect increases progressively with wavelength.
One approach to this problem is to reduce the angular difference by introducing, for example, a littrow prism. However, this prism introduces additional aberrations and losses, and generally degrades operation of the system in other ways.
Another solution to the problem has been proposed by Fastie.sup.1. It has been proposed that this anamorphic effect can be cancelled by using two mirrors with two different radii. Alternatively, identical mirrors and differing angles may be used.
.sup.1 See Optical Engineering, January/February 1974, Volume 13, No. 1, pages 25 et seq.
Yet another approach was proposed by Schaeffer.sup.2 who proposed the use of two toroidal mirrors. Generally, while this solution does provide a measure of improvement, it is expensive. Toroidal mirrors are extremely expensive to manufacture with the quality necessary for spectrographic applications. Furthermore, from an economic standpoint, toroidal mirrors can only be manufactured with relatively high tolerances (e.g. .lambda./2 as compared to the .lambda./10 of conventional mirrors) and relatively degraded performance is thus experienced. Nevertheless, Schaeffer's solution, which involves a precise relationship between the four transverse and sagittal focal lengths of both of the toroidal mirrors, does offer a limited degree of performance which is acceptable in some applications.
It is an object of the present invention to improve the operation of the Czerny-Turner monochromator, in particular, by the use of a single toroidal mirror in the collimating mirror position in an asymmetric design, without the coma and large astigmatism associated with symmetrical Czerny-Turner designs and with better cost and performance characteristics than the Schaeffer or Fastie approach. Likewise, the added cost of an additional toroid is avoided.
2 See Applied Optics, Jan., 1967, Volume 6, No. 1, pages 159 et seq.
Thus, the invention is aimed at maintaining costs of the system within a reasonable range while obtaining improved performance.